I need to get an inspection but can't until I solve the Service Engine Soon problem. Went to Auto Zone and had them read the codes. Two came up. PO400 & PO431. With the codes came probable causes. The 400 is related to the EGR system, 431 is related to the catalytic converter or H02S. Is there a way for me to pin point whats faulty? I would like to solve or do as much as I can before handing it over to a mechanic.

 

Hi:

(Posting your mileage would help with the diagnosis)

Not sure about the 300 code, but 431 is almost certainly a failed driver's side cat (small pre-cat attached to the exhaust manifold) There are serveral prior threads on this forum about 431 and 421 (passenger side pre-cat) You can use the Search funciton above to find them.

O2 sensors will not cause 431 or 421.

 

I would not rule out a failed O2 sensor. Here is a line from the FSM for code P0431:

TROUBLESHOOTING HINTS (The most likely
causes for this code to be set are: )
• Left bank side catalytic converter deteriorated.
• Left bank heated oxygen sensor failed.
• PCM failed.

Check/replace the O2 sensor first. Good luck

 

That's good input, HunterD, but there's a reason failed Cats are at the top of the list. It would still be helpful to know the mileage but the pre-cats quite often fail. As can be seen below there have been many posts on this. The fact that there are no accompanying O2 errors is a big clue. Trying a new Sensor will likely be money wasted. And the cheap generic sensors have been known to cause problems in Mitsu's (search for that as well) So the safest trial is to get OEM sensors for $200 ea??

I cut and pasted below some info from threads on the P0431 & 432 for convenience:


A.
This is the test sequence from OBDII reference manual: Note in particluar the sentence in red.

Error Codes:
P0421 Catalyst Efficiency below Threshold Bank 1
P0432 Catalyst Efficiency below Threshold Bank 2

Monitor Execution
Once per Driving Cycle. (On MIL equipped vehicles this test must be failed three times before the MIL light is switched on.)

Monitor Sequence
The following tests must be complete before the Catalyst Monitor can run:

• HO2S monitor tests complete and no DTCs
• No Fuel Trim DTCs
• EGR Monitor Complete
• No Misfire DTCs
• No SAIR (Secondary Air Pump) stuck on DTCs (where fitted.)

It should be noted that faulty HO2S sensors do not produce the DTCs P0431 and P0421. The sensors must all be tested and shown to be reliable before the Catalyst Monitor can be called by the Executive.


I had both 431 and 421 episodes in the last year and both times it was pre-cat failure and all the O2 sensors were fine. The first time I took it to a dealer to verify this with their full system scanner and they said not to bother because 431 and 421 are always failed cats. And they were right.

B.

Manybrews
Senior Member
Posts: 1,187
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You will NOT get codes for failed cats due to O2 sensors. In fact, the fact that you get cat. codes pretty much ASSURES that the O2 sensors are functioning PERFECTLY. Also, there are no fewer than 8 codes for the O2 sensors themselves which will pop up if you do actually have problems with them.



I hope this helps.

Ron

 

Thanks for all of the advice guys. I also talked to a couple of mechanics about this. They say the same as you jrmdir. Its got to be the cat. Looking online for after market cat. So far Discount Cat & Auto parts Warehouse seem to be the best price . Will update the out come of these problems. Oh, I have 188,000 miles on the Montero Sport. Thanks again guys.

 

jrmdir that is a very good detailed description. You obviously know what you are talking about. I always thought that a failed O2 sensor could give a reading that would be not matching (in vs. out) and therefore a Cat converter code may show. But you are right - it would be accompanied by a failed O2 sensor code as well. Especially that looking at the test sequence - O2 sensor malfunction is excluded first before calling up a P0421 and P0431 codes. Thanks for clarification on that.
One question/note I read in several places that generic cat converters do not work well with Monteros. Do you have experience with that? Would you suggest going with Mitsubishi or a generic is OK? Thanks